1. Field of the Invention
The invention relates to a hydrogen-storing alloy electrode and a secondary cell using the same.
2. Description of the Related Art
The hydrogen-storing alloy has been drawing attention as an energy conversion material and as an energy storage material, since it can store hydrogen safely and easily. Further, there is a great demand for the nickel-hydrogen secondary cell using the hydrogen storing alloy for the negative electrode, for consumer use, because of its characteristics such that it has a high capacity and that it is cleaner than the cells using lead or cadmium.
Specifically, the hydrogen storing alloys of this type are alloys of a metal element that can form a stable compound with hydrogen (any of Pd, Ti, Zr, V, rare-earth elements, alkaline-earth metal elements, etc.) and another metal element, and classified into AB5 type, AB2 type, etc. according to the crystal structure. In these crystal structures, the A site is occupied by the former metal element and the B site is occupied by the latter metal element. When the A site is occupied by a rare-earth element, the alloy is also called rare-earth alloy.
Generally, AB5-type rare-earth alloys such as LaNi5 are used for the nickel-hydrogen secondary cell. However, the discharge capacity of this type of cell is already higher than 80% of the theoretical capacity, so that there is a limit to enhancing the capacity further. Meanwhile, the rare-earth magnesium alloy obtained by replacing the rare-earth element in the AB5-type rare-earth alloy partly with Mg has a characteristic that it can store a larger amount of hydrogen gas than the AB5-type rare-earth alloy at temperatures close to the room temperature. Hence, the development of the secondary cell using the rare-earth magnesium alloy has been advanced.
For example, Japanese Unexamined Patent Publication No. 2000-73132 discloses a secondary cell using a rare-earth magnesium alloy. However, this rare-earth magnesium alloy is low in alkaline resistance. Hence, the secondary cell using this alloy solely has a problem that the cell life is short.
This problem comes from the fact: magnesium in the rare-earth magnesium alloy corrodes by reaction with an alkaline electrolyte, which decreases the negative-electrode capacity, and this corrosion reaction also consumes the alkaline electrolyte that should contribute to the cell reaction, so that the internal resistance of the cell gradually increases.
This consumption of the alkaline electrolyte is more prominent when the secondary cell has higher volume energy density. The reason is: in such cell, since the volumetric proportion of the positive electrode relative to the cell is increased, the amount of the alkaline electrolyte is originally smaller. Hence, decrease of the alkaline electrolyte easily leads to increase in internal resistance.
The inventor has developed a rare-earth magnesium hydrogen-storing alloy having high alkaline resistance and made a secondary cell using this. While this secondary cell has a longer cell life, another problem has been found: the cell that was charged continuously and then discharged shows a decreased discharge capacity after the next charging (referred to as “discharge capacity in re-discharging”). In other words, the cell has an inferior continuous-charging characteristic.